This invention relates to silicone resins and, in particular, to MQ resins and their use as tackifying components in pressure sensitive adhesive (PSA) compositions.
Silicone-based PSA compositions are well-known materials and are widely used where good adhesive properties over a wide operating temperature range are required such as a label on the cylinder wall of an internal combustion engine or on an internal wall of a freezer compartment. These compositions are typically provided as blends of high molecular weight linear polydimethyl siloxane gums endcapped by silanol groups and tackifying, usually partially hydrolyzed, MQ resins derived, e.g., from the hydrolysis of trialkylchlorosilanes with tetraalkyl orthosilicates and/or aqueous dispersions of sodium silicate. A frequent variation in these compositions is the use of linear silicone gum bearing diphenylsiloxane or methylphenylsiloxane in place of some dimethylsiloxane. Because of the high molecular weight of both the silicone gum and the MQ tackifying resin, it is necessary that silicone-based PSAs be packaged and applied as solutions in aromatic or aliphatic hydrocarbon solvents. Silicone-based PSAs are normally cured (crosslinked) to improve adhesive properties by the addition of heat-activated organic peroxide curing catalysts, benzoyl peroxide (BPO) being commonly used for this purpose.
Since all components of a conventional silicone-based PSA composition are very soluble in, and are packaged and used in, aromatic and aliphatic hydrocarbon solvents, it is not surprising that even BPO-crosslinked silicone-based PSAs are readily attacked by these solvents. Solvents cause the adhesive to swell and to delaminate (lift off) from substrates to which they are applied.
There are certain silicone-based PSA applications for which a high level of solvent resistance is particularly desirable. Barcode labeling of various medical samples including biopsied tissue, blood, urine and other diagnostic materials is widely practiced by health care providers in hospital and other institutional settings in order to prevent loss of critical samples or their being confused with a different sample from other sources. Tissue samples, in particular, are packaged in porous thermoset plastic containers that permit free flow of dyestain solution, various solvents including alcohols and aromatic solvents, formalin, and other chemicals necessary to treat tissue samples for purposes of preservation and analysis. The barcode label is typically generated and affixed to the outside of the tissue-holding vessel prior to whatever sequence of washing, dyestain or other preparation is carried out. The entire sample holder is immersed for periods of time in a variety of aggressive media for up to hours at a time. It is critical that the adhesive securing the barcode label to the tissue container does not fail, i.e., does not lift off or delaminate, during these immersions. Similarly, thin sections of tissue sample are affixed to glass microscope slides for analysis; barcode labels applied to the glass slide must also resist attack by aggressive solvents and other diagnostic reagents. While the adhesive properties of silicone-based PSAs are well suited for use with barcode labels on glass and plastic substrates, it is highly desirable that the PSAs in these applications possess a high degree of solvent-resistance.
Fluoroalkylsilicone rubbers and coatings are well known for their good solvent resistance properties. Photocurable and thermally curable solvent-resistant materials of this type that are intended for protective and conformal coating applications are known from amongst others, U.S. Pat. Nos. 5,178,959; 5,616,403; 5,753,318; 5,919,526; and, 6,074,703.
U.S. Pat. No. 5,436,303 describes solvent-resistant PSAs containing blends of fluorosilicone polymers, silanol-containing MQ tackifier resins and resin-compatible diorganosiloxane polymers. These blends, while useful, are difficult to prepare and store since the highly fluorinated fluorosilicone polymers, or gums, employed therein are immiscible with conventional dimethylsilicone polymers and gums. Furthermore, the MQ resins in these blends, which are required to make the adhesive tacky, are also incompatible with fluorosilicone polymers. It is a matter of considerable difficulty to provide a solvent mixture in which all the polymeric components of the adhesive composition will remain soluble over the expected storage life of the product and after its application.
It is therefore desirable that a silicone-based PSA composition be provided that possesses good solvent resistance but is easily processed and stable both before and after application.